PLC: Lights don't turn on after a power outage

ProblemThis problem occurs if the first trigger after midnight is an OFF trigger. If the PLC loses power between midnight and this trigger, it will not turn on the lights.

For example, a PLC is programmed as follows:

Trigger 1 03:00 OFFTrigger 2 18:00 ON

A power failure occurs between 00:00 and 03:00 and recovers immediately. When the PLC turns on it looks for the last trigger before the current time and uses it. However, the PLC views 00:00 as the beginning of the day and will not look before that time.

In this example, the PLC will not find an ON trigger between 00:00 and 03:00 and therefore not turn on the lights.

SolutionIf you have a light program that calls for the light to be on before midnight and off after midnight, use an extra trigger at 00:00 set to ON.

In addition, this extra trigger must be the FIRST trigger since the PLC requires you to program the triggers in sequential order from 00:00 to 23:59.